I followed the link as you suggested. I found a slight correction on the way it works.
A “shadow account” was some layperson’s attempt to describe what happened. That seemed clear to me immediately. It also seems that Threads and Instagram are much more intertwined than users expect.
I understand why this would upset people! I was furious when I tapped one screen wrong and connected my Facebook and Instagram accounts. It can’t be undone. It changed a profile picture. I didn’t quite become angry enough to delete both, but I stopped using them.
Yes, “people of color” is considered respectful.
And you’re right, language can change pretty fast. I’ve seen plenty of respectful words become slurs. I’ve even seen slurs be reclaimed by communities. Don’t even get me started on person-first vs identity-first language.